From f64ea9979a5eaddaed98bde17832f855f2f0daee Mon Sep 17 00:00:00 2001
From: Karel Zak <kzak@redhat.com>
Date: Thu, 2 Nov 2023 10:41:03 +0100
Subject: [PATCH] libmount: improve mnt_table_next_child_fs()

The function utilizes the struct libmnt_itr to iterate through the mountinfo file
but neglects the direction specified by the iterator. This a bug. The application
must manage the direction, as, for instance, umount(8) requires the children of
the mountpoint in reverse order.

Fixes: https://github.com/util-linux/util-linux/issues/2552
Signed-off-by: Karel Zak <kzak@redhat.com>
---
 libmount/src/tab.c | 25 ++++++++++++++++++-------
 1 file changed, 18 insertions(+), 7 deletions(-)

diff --git a/libmount/src/tab.c b/libmount/src/tab.c
index 0d5c115..be0c13a 100644
--- a/libmount/src/tab.c
+++ b/libmount/src/tab.c
@@ -649,8 +649,8 @@ int mnt_table_get_root_fs(struct libmnt_table *tb, struct libmnt_fs **root)
  * @parent: parental FS
  * @chld: returns the next child filesystem
  *
- * Note that filesystems are returned in the order of mounting (according to
- * IDs in /proc/self/mountinfo).
+ * Since version 2.40, the filesystems are returned in the order specified by
+ * @itr. In the old versions the derection is always MNT_ITER_FORWARD.
  *
  * Returns: 0 on success, negative number in case of error or 1 at the end of list.
  */
@@ -659,6 +659,7 @@ int mnt_table_next_child_fs(struct libmnt_table *tb, struct libmnt_iter *itr,
 {
 	struct libmnt_fs *fs;
 	int parent_id, lastchld_id = 0, chld_id = 0;
+	int direction = mnt_iter_get_direction(itr);
 
 	if (!tb || !itr || !parent || !is_mountinfo(tb))
 		return -EINVAL;
@@ -676,7 +677,7 @@ int mnt_table_next_child_fs(struct libmnt_table *tb, struct libmnt_iter *itr,
 
 	*chld = NULL;
 
-	mnt_reset_iter(itr, MNT_ITER_FORWARD);
+	mnt_reset_iter(itr, direction);
 	while(mnt_table_next_fs(tb, itr, &fs) == 0) {
 		int id;
 
@@ -690,10 +691,20 @@ int mnt_table_next_child_fs(struct libmnt_table *tb, struct libmnt_iter *itr,
 		if (id == parent_id)
 			continue;
 
-		if ((!lastchld_id || id > lastchld_id) &&
-		    (!*chld || id < chld_id)) {
-			*chld = fs;
-			chld_id = id;
+		if (direction == MNT_ITER_FORWARD) {
+			/* return in the order of mounting */
+			if ((!lastchld_id || id > lastchld_id) &&
+			    (!*chld || id < chld_id)) {
+				*chld = fs;
+				chld_id = id;
+			}
+		} else {
+			/* return last child first */
+			if ((!lastchld_id || id < lastchld_id) &&
+			    (!*chld || id > chld_id)) {
+				*chld = fs;
+				chld_id = id;
+			}
 		}
 	}
 
-- 
2.33.0